1. Field of the Invention
This invention is related to a method of measuring the density of water molecules in air, namely the humidity of air, and more particularly to a method of measuring humidity with high precision using two laser beams of different wavelengths at each of which the second differential derivative of the absorption spectra of water molecules is equal to zero.
2. Prior Art Statement
The instrument most commonly used for measuring humidity is the dry and wet bulb hygrometer. However, this and other types of hygrometers which rely on a physical phenomenon for measuring humidity are low in precision and difficult to use. What is more, they are unable to measure absolute humidity.
While theoretical studies have been made regarding measurement of humidity by an optical method and it is known that such a method has the potential of providing highly accurate measurement of absolute humidity, there have been very few practical proposal in this connection.
In contrast, in the field of pollution control, two optical methods have been developed and practically applied for the measurement of the concentration harmful gases etc. in the air.
In the first of these methods, the existence and concentration of gases and other substances in the air is determined by observation of the light absorption spectrum (the attenuation component) of the molecules contained in the air. With this method, even the components of mixed gases and trace molecules in the atmosphere can be identified from changes in the absorption spectrum. However, as concentration is determined on the basis of the light intensity of the spectrum, measurement error inevitably arises because of, for example, variation in the intensity of the light source and diffusion of the light. This makes high precision measurement difficult and necessitates various types of processing for compensation.
In the second method, concentration is calculated from the index of refraction of the gas. As this method employs an interferometer, it enables humidity measurement with very high resolution. However, where humidity measurement is to be carried out with respect to a mixed gas such as air, a problem arises as to how to estimate the indices of refraction of molecules other than water molecules and this complicates the post-measurement processing.